STRUCTURAL GEOLOGY 397 



only a few inches from crest to crest, but in rare instances they 

 attain much greater size, examples 30 feet across being known. 1 

 Occasional ridges and depressions of much greater dimensions are 

 produced which are attributable to the formation of successive 

 bars, or to the building of wave-cusps. 2 Rill-marks are not in- 

 frequently produced by the undertow and other currents passing 

 over pebbles, shells, etc. (Fig. 264). 



Sediments are sometimes exposed between tides, or under other 

 circumstances, for periods long enough to permit drying and 



Fig. 304. Raindrop impressions. (Brigham.) 



cracking at the surface. On the return of the waters, the cracks 

 may be filled and permanently preserved. These are known as 

 sun-cracks or mud-cracks (Figs. 267 and 268). They affect shales 

 chiefly, but are occasionally seen in limestones and fine-grained 

 sandstones. During the exposure of the sediments, a shower may 

 pass, and raindrop impressions (Fig. 304) be made which are sub- 

 sequently filled by fine sediment and preserved. 



Uncomformities. In Figs. 305, 306, and 307, one set of bed is 

 seen to be out of harmony with another set. This relation is one 



Gilbert, Bull. Geol. Soc. Am., Vol. X, 1898, pp. 135, 140. - 

 2 Branner, Jour. Geol., Vol. VIII, 1900, pp. 481-484. 



